1. Field of the Invention
The present invention relates generally to bone fixation screws and, more particularly, to a bone fixation screw of a dynamic construction and its unique method of fabrication which enables it to relax as the bone relaxes while continuing to draw the bone fragments together.
2. Description of the Prior Art
Upon fracture of a bone, the body's response is to stabilize and heal the fragments in a functional orientation. When a fragment or fragments are displaced in a non-functional or non-anatomical orientation, non-union or malunion can occur. In this instance, open reduction of the fracture may be necessary.
In open reduction, a surgical opening is made, the fracture fragments are realigned, and plates and screws, wire, or other hardware are added to secure the fracture fragments to the bones from whence they came. When a screw is used, whether to secure two fragments together or to secure a plate to the bone, and the screw is tightened, initially, tension in the screw is very high, and holds the fragments together. However, bone is a viscoelastic material and undergoes a phenomenon known as stress relaxation immediately after torque has been applied to the screw. The stress relaxation response is quite pronounced and causes immediate and rapid reduction in the screw tension and, hence, the force holding the fragments together. Furthermore, after a conventional screw is tightened, and the fragment is laterally displaced, as by bending, the rigidity of the screw causes the surrounding bone to fail since the bone is of lower strength and stiffness than the screw. This can lead to failure of fixation and eventual non-union or malunion. Typical constructions of known orthopedic fasteners are disclosed in U.S. Pat. No. 4,537,185 to Stednitz, No. 4,711,232 to Fisher et al, No. 4,227,518 to Aginsky, No. 3,554,193 to Constantinou, and No. 3,051,169 to Grath. In each of these instances, a standard rigid screw is employed which is certain to become loosened relatively rapidly due to the stress relaxation phenomenon previously mentioned.
Another prior art construction of interest in this regard is disclosed in U.S. Pat. No. 2,985,168 to Jonas et al which discloses a bone aligning or splint device which is used to assist in bringing the ends of a pair of fractured fragments of a long bone into alignment and to retain that alignment until the fracture has completely mended. The splint device includes an outer sleeve member received in the intramedullary canal of a first bone fragment with a pin slidably mounted in the sleeve member and a spring above the pin urging it outwardly of the sleeve member. The pin is held in its retracted position until the bone fragments are aligned, then is released under the bias of the spring for reception in the intramedullary canal of the second bone fragment When the splint is in its fully expanded position within the bone fragments, the fragments are in substantial alignment to enable mending to occur. While the Jonas et al splint is satisfactory for its intended purpose of aligning, and maintaining alignment of the bone fragments, it is not intended to, nor does it, actually hold the fragments together.